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.\" $Id: rtld.1,v 1.2 1995/10/05 05:16:52 nate Exp $
.\"
.\" Copyright (c) 1995 Paul Kranenburg
.\" All rights reserved.
.\"
.\" Redistribution and use in source and binary forms, with or without
.\" modification, are permitted provided that the following conditions
.\" are met:
.\" 1. Redistributions of source code must retain the above copyright
.\" notice, this list of conditions and the following disclaimer.
.\" 2. Redistributions in binary form must reproduce the above copyright
.\" notice, this list of conditions and the following disclaimer in the
.\" documentation and/or other materials provided with the distribution.
.\" 3. All advertising materials mentioning features or use of this software
.\" must display the following acknowledgement:
.\" This product includes software developed by Paul Kranenburg.
.\" 3. The name of the author may not be used to endorse or promote products
.\" derived from this software without specific prior written permission
.\"
.\" THIS SOFTWARE IS PROVIDED BY THE AUTHOR ``AS IS'' AND ANY EXPRESS OR
.\" IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE IMPLIED WARRANTIES
.\" OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE ARE DISCLAIMED.
.\" IN NO EVENT SHALL THE AUTHOR BE LIABLE FOR ANY DIRECT, INDIRECT,
.\" INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT
.\" NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
.\" DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
.\" THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
.\" (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE OF
.\" THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
.\"
.Dd June 27, 1995
.Dt RTLD 1
.Os FreeBSD
.Sh NAME
.Nm ld.so
.Nd run-time link-editor
.Sh DESCRIPTION
.Nm
is a self-contained, position independent program image providing run-time
support for loading and link-editing shared objects into a process'
address space. It uses the data structures
.Po
see
.Xr link 5
.Pc
contained within dynamically linked programs to determine which shared
libraries are needed and loads them at a convenient virtual address
using the
.Xr mmap 2
system call.
.Pp
After all shared libraries have been succesfully loaded,
.Nm
proceeds to resolve external references from both the main program and
all objects loaded. A mechanism is provided for initialisation routines
to be called, on a per-object basis, giving a shared object an opportunity
to perfrom any extra set-up, before execution of the program proper begins.
This is useful for C++ libraries that contain static constrictors.
.Pp
.Nm
is itself a shared object that is initially loaded by the startup module
.Em crt0 .
Since
.Xr a.out 5
formats do not provide easy access to the file header from within a running
process,
.Em crt0
uses the special symbol
.Va _DYNAMIC
to determine whether a program is in fact dynamically linked or not. Whenever
the linker
.Xr ld 1
has relocated this symbol to a location other then 0,
.Em crt0
assumes the services of
.Nm
are needed
.Po
see
.Xr link 5
for details
.Pc \&.
.Em crt0
passes control to
.Nm
\&'s entry point before the program's
.Fn main
routine is called. Thus,
.Nm
can complete the link-editing process before the dynamic program calls upon
services of any dynamic library.
.Pp
To quickly locate the required shared objects in the filesystem,
.Nm
may use a
.Dq hints
file, prepared by the
.Xr ldconfig 8
utility, in which the full path specification of the shared objects can be
looked up by hashing on the 3-tuple
.Ao
library-name, major-version-number, minor-version-number
.Ac \&.
.Pp
.Nm
recognises a number of environment variables that can be used to modify
its behaviour as follows:
.Pp
.Bl -tag -width "LD_TRACE_LOADED_OBJECTS"
.It Ev LD_LIBRARY_PATH
A colon separated list of directories, overriding the default search path
for shared libraries.
This is ignored for set-user-ID and set-group-ID programs.
.It Ev LD_PRELOAD
A colon separated list of shared libraries, to be linked in before any
other shared libraries. If the directory is not specified then
the directories specified by LD_LIBRARY_PATH will be searched first
followed by the set of built-in standard directories.
This is ignored for set-user-ID and set-group-ID programs.
.It Ev LD_WARN_NON_PURE_CODE
When set, issue a warning whenever a link-editing operation requires
modification of the text segment of some loaded object. This is usually
indicative of an incorrectly built library.
.It Ev LD_SUPPRESS_WARNINGS
When set, no warning messages of any kind are issued. Normally, a warning
is given if satisfactorily versioned library could not be found.
.It Ev LD_TRACE_LOADED_OBJECTS
When set, causes
.Nm
to exit after loading the shared objects and printing a summary which includes
the absolute pathnames of all objects, to standard output.
.El
.Pp
.Sh FILES
/var/run/ld.so.hints
.Pp
.Sh SEE ALSO
.Xr ld 1
.Xr ldconfig 8
.Xr link 5
.Sh HISTORY
The shared library model employed first appeared in SunOS 4.0
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